The invention relates to a method of pressing an article of a thermoplastic material in which a charge of molten material is deposited in a mould and the central axis of the mould is brought into alignment with the axis of a pressing plunger, after which the plunger and the mould are moved towards one another to carry out a pressing operation while their axial alignment is maintained.
The invention also relates to apparatus for manufacturing an article of a thermoplastic material by pressing, which apparatus comprises a frame, at least one mould, and a plunger which can cooperate with the mould and which is secured to a plunger support which is supported in a guide mechanism and which is axially movable by a drive means.
Such method and apparatus are disclosed in British Patent Specification 1,105,475.
The known method aims at maintaining the axial alignment of the plunger and the mould throughout the entire pressing operation. In this respect the plunger and the mould (the tools) are not allowed to move freely relative to one another until the plunger and the mould have reached the extreme pressing position and the plunger has been extracted so far from the mould that the plunger is no longer in contact with the pressed article.
As the charge of a thermoplastic material supplied to the mould is generally neither of a wholly symmetrical shape nor situated exactly in the centre of the mould, it frequently happens that towards the end of the pressing operation there is still a substantial flow of material in the space between the plunger and the mould. However, as the thermoplastic material cools its resistance to flow increases. In the known method this gives rise to the problem that at the end of the pressing operation the plunger and the mould are subjected to substantial transverse forces which may lead to uncontrolled stresses in and deformation of the tools. Moreover, the deformation of the tools may give rise to flaws, such as cracks, in the pressed article, in particular after the extreme pressing position of the plunger and the mould has been reached.
The known apparatus is provided with a mechanism, arranged between the plunger and the drive means, for aligning the plunger and the mould during the pressing stoke. The mechanism comprises two paralel plates, of which one is secured to the plunger support and the other to the drive means. A plurality of balls arranged between the plates permits a limited relative movement of the plates. The mould, which can cooperate with the plunger, is provided with an inwardly bevelled annular wall for positioning and guiding the plunger. The plunger is positioned as it moves towards the mould. The plunger first butts against the annular wall and subsequently slides along the bevelled inner side of this wall, the plunger thereby being moved in a transverse direction until the plunger and the mould are aligned.
A disadvantage of the known apparatus is that during the alignment of the plunger and the mould substantial forces occur at the contact surfaces between the plunger and the annular wall of the mould. This may lead to premature wear to the plunger and the mould and hence to a short life of these parts of the apparatus. Another disadvantage is that the annular wall which constantly bears on the mould is subject to substantial temperature variations so that an acurate alignment of the plunger relative to the mould is not possible due to the expansion and contraction of the wall.